The present invention relates to an electronic ballast for powering a discharge lamp and an illumination fixture incorporating the same.
Referring to FIG. 12, an example is shown of a discharge lamp ballast for powering a hot cathode type discharge lamp such as a high-pressure discharge lamp, also called an HID (High-Intensity Discharge lamp). The discharge lamp ballast 1 includes a DC power source E for converting an AC power input, as for example supplied from an AC power source AC such as a commercial power source, into direct-current (DC) power.
The DC power source E includes a diode bridge DB with an output terminal on the low voltage side connected to ground and that performs full wave rectification of the AC power input, a diode D0 having an anode coupled to an output terminal on the high voltage side of the diode bridge DB via an inductor L0 and a cathode coupled to ground via an output capacitor C0, a switching element Q0 having a first end coupled to a node between the inductor L0 and the diode D0 and a second end coupled to ground, and a drive circuit (not shown in the drawing) for controlling the switching element Q0 to turn on or off to maintain a constant output voltage of the DC power source E, that is, a voltage across the output capacitor C0. Specifically, the DC power source E is configured by connecting a commonly-known boost converter (a step-up chopper circuit) between the output terminals of the diode bridge DB.
In addition, the discharge lamp ballast 1 as previously known in the art includes a full bridge circuit including four switching elements Q1 to Q4 as a power converter for converting DC power input from the DC power source E into AC power. Field effect transistors (FETs or MOSFETS) can be used as the switching elements Q1 to Q4. One of the output terminals of the above-mentioned full bridge circuit, that is, a node between the switching elements Q1 and Q2, is coupled to one end of a discharge lamp La (that is, one of the filaments) via an inductive element which in the example shown includes a primary and a secondary winding of an auto transformer AT. Additionally, a tap on an auto-transformer AT between the primary and secondary windings is coupled to ground via a first capacitor C1. The other output terminal of the full bridge circuit, that is, a node between the switching elements Q3 and Q4, is coupled to the other end (that is, the other filament) of the discharge lamp La via the inductor L1. A second capacitor C2 is coupled on a first side between the switching elements Q1 and Q2 and on a second side between the inductor L1 and the discharge lamp La. The auto-transformer AT, the first capacitor C1, the second capacitor C2, and the inductor L1 collectively define a resonant circuit coupled between the output terminals of the power converter (hereinafter referred to as “a load circuit”) together with the discharge lamp La.
The discharge lamp ballast 1 further includes a control circuit 2 for driving each of the power converter switching elements Q1 to Q4 respectively. The control circuit 2 turns the switching elements Q1 to Q4 on and off so that a diagonally-positioned pair among the switching elements Q1 to Q4 (i.e., Q1 and Q4 or Q2 and Q3) can be turned on at the same time and a pair connected with each other in series among the switching elements Q1 to Q4 (i.e., Q1 and Q2 or Q3 and Q4) can be alternately turned on or off. In this manner, the DC power input from the DC power source E is converted into AC power, and an AC power frequency with polarity reversal is generated by the above-mentioned on-off driving (hereinafter referred to as “an operational frequency”).
During a startup procedure of a discharge lamp La by the discharge lamp ballast 1, the control circuit 2 carries out three operations. First, a startup operation is conducted to ignite the discharge lamp La by relatively increasing an output voltage from the power converter. Second, a filament heating operation is conducted wherein an output power frequency from the power converter is relatively increased to heat each filament of the discharge lamp La. Third, a normal (i.e., steady-state) operation is conducted to output an AC signal from the power converter as needed to maintain a stable light output from the discharge lamp La.
With reference to FIG. 13, an example of operation of the control circuit 2 as previously known in the art may be explained in detail. The first four waveforms in FIG. 13 show input drive signals to the respective switching elements Q1 to Q4, that is, voltages applied between the gate and the source of each respective switching element, with respect to time. The respective switching elements Q1 to Q4 are turned on in periods when the above-mentioned drive signals are in an H level and turned off in periods when the above-mentioned drive signals are in an L level.
When the power source is turned on, the control circuit 2 first conducts a startup operation to initiate discharge in the discharge lamp La. During the startup period P1 when the startup operation is carried out, the control circuit 2 sufficiently raises a voltage output Vla to the discharge lamp La (hereinafter referred to as “a lamp voltage”) to initiate discharge in the discharge lamp La by setting the operational frequency approximately to a resonant frequency of the load circuit consistent with a condition where the discharge lamp La is producing no light output or otherwise turned off (herein referred to as a “pre-ignition resonant frequency”) which may be, for example, a few dozen kHz to a few hundreds kHz. That is, the operational frequency in such a condition is set to approximately a resonant frequency (or 1/n multiplied by the resonant frequency, where n is a whole number) of a resonant circuit which includes a primary winding of the auto transformer AT coupled between the switching elements Q1 and Q2 and the first capacitor C1. When the lamp voltage Vla is raised to a voltage required for ignition, that is the start of glow discharge, the discharge lamp La ignites and an output current begins flowing through the discharge lamp La. The auto transformer AT and the first capacitor C1 may further be referred to as a starting circuit.
After the above-mentioned startup period P1, the control circuit 2 shifts to a filament heating period P2 during which a filament heating operation is carried out. In the example of FIG. 13, the operational frequency during the filament heating period P2 is maintained at the same frequency as the operational frequency during the start-up period P1.
After the filament heating operation is carried out for what may be a predetermined time, the control circuit 2 shifts to a normal period P3 during which steady-state operation is carried out. As the temperature in the discharge lamp La rises, the lamp voltage V gradually rises for a few minutes immediately after the shift to the normal period P3 and then stabilizes. The operational frequency f during the steady-state operation is, for example, a few dozen Hz to a few hundreds Hz. In the example of FIG. 13, the control circuit 2 in the normal period P3 controls output power to the discharge lamp La with a PWM control to turn on or off one of the switching elements Q3 and Q4, whereas switching element Q3 is controlled when switching element Q2 is on and switching element Q4 is controlled when switching element Q1 is on, at a duty ratio depending on a power to be output to the discharge lamp La and at a sufficiently higher frequency than the operational frequency f.
In the example of FIG. 13, because the same operational frequency is employed in the startup period P1 and in the filament heating period P2, an amplitude of the lamp current 11a is smaller than an amplitude. It required to sufficiently heat a filament of the discharge lamp La.
Referring now to FIG. 14, it has been proposed to decrease the operation frequency f in a transitional phase. Because the operation frequency f is in a range where an amplitude |Ila| of the lamp current Ila decreases with respect to the operation frequency f (as shown in FIG. 15), the control circuit 2 decreases the lamp voltage Vla to increase the lamp current Ila in the filament heating period P2 by setting the operation frequency f to a first predetermined frequency (f1pred) which is lower than the operation frequency f upon termination of the startup period P1. In this manner, the amplitude of the lamp current Ila in the filament heating period P2 can be sufficiently increased, and the discharge in the discharge lamp La can be shifted from glow discharge to arc discharge and stabilized. Each of the filaments of the discharge lamp La is heated, and accordingly an asymmetric current condition which is caused by a temperature difference between the filaments of the discharge lamp La is also decreased after the filament heating period P2.
Referring again to FIG. 14, the control circuit 2 gradually increases the lamp voltage Vla during the startup period P1 by gradually decreasing the operation frequency f to approach a pre-ignition resonant frequency (fres). The operation frequency f is further decreased from the first predetermined frequency f1pred in the middle of the filament heating period P2 to a second predetermined frequency f2pred. 
In a discharge lamp ballast so described, the discharge lamp La shifts from glow discharge to arc discharge during a filament heating operation and stabilizes after transition to a normal operation, in comparison to a case where the filament heating operation is not conducted, and preventing the ballast from suddenly becoming extinguished.
However, the impedance of a load circuit varies in accordance with characteristics of the circuit components and of the discharge lamp La, and further due to ambient temperature. Accordingly, when values for the operation frequency f in the filament heating operation are predetermined, lamp current may be too low in the filament heating operation and the light output from the discharge lamp La in the subsequent normal operation is therefore not stabilized, or conversely an excessive lamp current may flow and undesirable electric stresses may be applied to the circuit components and the discharge lamp La.